In recent years, higher integration of semiconductor devices has been required with enhancement in the performance and miniaturization of electronic appliances such as cell phones and IC cards. As a procedure for higher integration, the finer pattern of semiconductor elements themselves, and a stack structure in which semiconductor elements are stacked longitudinally have been investigated. In production of the stack structure, an adhesive is used for junction between the semiconductor elements.
In production of the stack structure, flip chip bonding has been known as a method of mounting an IC chip on a substrate. This is a method in which a plurality of bumps (projected terminals) are disposed and electrically connected to electrode terminals on the substrate. An underfill agent is used to seal a gap between the substrate and the IC chip and protect the IC chip against moisture and an exterior stress. As the underfill agent, a composition containing an epoxy resin has been used (e.g., Patent Documents 1 and 2). Conventionally, a method in which an underfill agent is injected after connection of a substrate and an IC chip to bumps (post-applied underfilling), and then thermally cured has been often used. However, miniaturization of bumps, a decrease in pitches, and an increase in the size of IC chips make the post-applying of an underfill agent difficult. As a countermeasure, a pre-applied underfilling will be mainly employed in the future, in which an underfill agent is previously formed on a wafer with bumps, the wafer is diced, and the diced IC chip is subjected to flip chip bonding.
In such flip chip bonding, bumps are connected readily after bonding of underfill. Therefore, a conventional adhesive described in Patent Document 3 makes it difficult to prevent generation of voids. This is because the adhesive is momentarily exposed to high temperature in an uncured state.
A wafer-on-wafer (WOW) structure in another method of producing a stack structure described in Patent Document 4 makes it difficult to employ a low molecular weight component that causes voids and a component that has low heat resistance and is decomposed. This is because the WOW structure requires adhesion between wafers without generation of voids.
A thermosetting resin composition containing a compound having an aromatic benzoxazine ring as a main chain has been known (e.g., Patent Documents 5 and 6). A benzoxazine resin having excellent heat resistance and electrical characteristics has been also known. The thermosetting resin composition containing a compound having an aromatic benzoxazine ring as a main chain described in Patent Documents 5 and 6 has low crosslink density and high linear expansion coefficient at a temperature equal to or higher than the glass transition point. Therefore, there is a concern of reduction in reliability caused by occurrence of cracks and peeling due to thermal history.
As a known technique, a technique of mixing an epoxy resin in a benzoxazine resin to improve the crosslink density has been known. However, a cross-linking reaction of the mixed resin is slow, and sufficient crosslink density is not obtained. As a countermeasure, a technique of adding an epoxy resin curing agent, which is typified by an imidazole-based compound, has been widely known. However, since the epoxy resin curing agent has low heat resistance, there is a concern of reduction in reliability caused by generation of voids.